In recent years, with increasing in the degree of integration and precision of semiconductor devices, quality requirements for semiconductor crystal substrates have tended to become stricter. Semiconductor crystals are primarily produced by the CZ method, and every effort has been made to produce crystals having higher purity, lower defect density, and higher uniformity. Recently, it has been found that crystal defects are closely related not only to purity of a raw material, purity of a member used, and precision of an apparatus, but also to the thermal history of a crystal during its growth. For example, for silicon, thermal history has an effect on OSF (Oxidation Induced Stacking Faults), oxygen precipitation, BMD (Bulk Micro-Defect), FPD (Flow Pattern Defect), LSTD (Laser Scattering Tomography Defect), and the oxide dielectric breakdown voltage. Also, for compound semiconductors such as GaP, GaAs, and InP, thermal history has a considerable effect on dislocation density and the level of such defects as functioning as a donor or an acceptor. Accordingly, there have been proposed crystal-producing apparatuses having a variety of furnace structures that control defects in crystals through adjustment of thermal history during crystal growth (refer to, for example, H. Yamagishi, I Fusegawa, K. Takano, E. Iino, N. Fujimaki, T. Ohta, and M. Sakurada, Proceedings of the 17th International Symposium on Silicon Materials Science and Technology, SEMICONDUCTOR SILICON 1994, PP.124-135).
However, according to the proposed apparatuses or methods, only the temperature at a certain position within a furnace can be increased or decreased, i.e. the entire temperature distribution within a furnace cannot be adjusted. Further, this control of temperature at a certain position within the furnace is such that the temperature of an entire grown crystal is increased or decreased. That is, this temperature control has no degrees of freedom, and its accuracy is poor. In addition, controlling a temperature only in a specific temperature region is extremely difficult. Also, when a temperature distribution must be changed in order to meet a new design requirement, an apparatus must be redesigned from the beginning.
The present inventors found that when the heat insulating cylinder surrounding a crucible and a heater used in the CZ method has a structure such that it is vertically divided so that a gap is formed between the divisions, the thermal history of a crystal and a temperature distribution within a furnace can be controlled, thereby solving the above-described problems. Based on this finding, the present inventors have proposed an improved technique disclosed in Japanese Patent Application No. 7-143586.
However, the results of further experiments conducted by the inventors show that although the above invention permits precise control of the thermal history of a crystal and a temperature distribution within a furnace in most temperature regions, when a crystal must be grown to have a thermal history such that the cooling rate is sufficiently decreased in a specific temperature region, a controllable temperature region is unexpectedly narrow due to insufficient heat retaining capability.
Recently, it has also been reported that crystal defects induced during crystal growth disappear when the cooling rate is sufficiently decreased in a specific temperature region (refer to Fujimaki et al., "Effect of Micro-Defect in a Single Crystal of Silicon on Oxide Film," UCS Semiconductor Substrate Technology Workshop, ULTRACLEAN TECHNOLOGY, Vol. 7, Issue 3, pp. 26-). Thus, there has been demand for an apparatus for producing crystals in accordance with the CZ method in which only a specific temperature region is cooled accurately at a sufficiently decreased rate.
The present invention has been accomplished to solve the above-described problems, and an object of the present invention is to provide an apparatus and method for producing crystals whereby the thermal history during crystal growth according to the CZ method can be controlled with ease and accuracy, particularly to provide an apparatus and method for producing crystals whereby the cooling rate in a specific temperature region can be decreased sufficiently.